Portable electronic devices are increasingly being powered by rechargeable batteries. This is also the case for miniature hearing devices adapted to be worn at an ear or at least partially within an ear canal of a user. Such hearing devices include ear phones, communication devices, hearing aids (also referred to as hearing prostheses or hearing instruments) for hard of hearing people or hearing enhancement devices for augmenting the hearing capability of normal hearing persons, as well as hearing protection devices designed to prevent noise-induced hearing loss. Due to the small size of such hearing devices the batteries employed therein are tiny and therefore often difficult to handle, e.g. when replacing a depleted battery with a new one, especially for elderly users with reduced dexterity. Hence, the use of rechargeable batteries which do not need to be removed from a device for recharging represent a considerable improvement for such users. Furthermore, such hearing devices are typically utilised for prolonged periods of time, e.g. during most of the day on a daily basis, so the batteries need to be replaced very often, for instance every few days. This puts a substantial financial burden on the users of such devices due to the considerable cost of the batteries required for operating them.
Only recently have battery technologies evolved to the point where the capacity of a small rechargeable battery cell, such as a size 10A battery is sufficient to operate hearing devices continuously for a number of days. The limited space available and the prolonged runtime of hearing devices place formidable requirements on the type of rechargeable batteries applicable therein. Ideally for extended wear hearing devices, a suitable rechargeable battery should feature the capability to provide small currents, e.g. on the order of 40 μA, for 10 to 12 days, i.e. have a capacity of 10 to 12 mAh, and to be chargeable with high currents in order to minimise the time for recharging the battery. Presently, NiMH (Nickel-Metal Hydride) batteries are a good candidate for use in such hearing devices. For applications where the battery cannot be removed from the hearing device, long battery life in terms of a high number of recharging cycles is an important prerequisite. Factors which adversely impact battery life include deep discharge as well as excessive overcharging. Both can result in a permanent loss of capacity in a NiMH battery cell. Therefore, the employed charging algorithm has a considerable influence on battery life time. The goal is typically to minimise the time it takes to recharge the battery, whilst ensuring that maximum charge is stored in the battery, i.e. that the battery is filled to its maximum capacity, and at the same time maximising battery longevity by maintaining a high battery capacity over a large number of recharging cycles. Consequently, there is a need for efficient charging methods for small NiHM batteries utilised in hearing devices.
EP 2 259 404 A1 discloses a charging algorithm for NiMH batteries, which (re-)charges the battery in a relatively short time and in a relatively gentle manner with regards to the battery's life time degradation, and which may be used for listening devices such as hearing instruments. According to the teaching of EP 2 259 404 A1 the charging process is not deliberately terminated, e.g. after a specific charging time, and may continue after the battery has reached its full capacity. This bears the risk of overcharging which in turn can result in a reduced battery life.